Whether you are walking through a hardware store, looking
under the hood of your car, or mowing the grass, you will find zinc plating at
work protecting steel from corrosion in the products we use every day.Zinc plating has found wide acceptance as a
surface finish throughout all consumer, industrial, and commercial
products.Although it is very common in
our daily lives, few of us have paused to contemplate this important
engineering finish, much less understand how it works.

Zinc is a bluish-white metal, which, if mechanically
polished, or electrodeposited with appropriate brighteners, somewhat resembles
chromium in appearance.However, the
reflectivity of the polished surface is soon lost in most atmospheres.8This quick tarnishing and corroding,
is the property that makes zinc plating work so well in providing“sacrificial” protection for
steel.To learn more about the
sacrificial nature of zinc plating, please read the section on “Galvanic
Series of Metals in Seawater” elsewhere in this report.

The relatively low cost, protective nature,
and attractive appearance of zinc plating make it a popular coating for nuts,
bolts, washers, metal stampings, and automotive components, fabricated parts
for industrial applications, and also serves as an effective undercoat for
paints.8

Electrolytic zinc coatings are used to protect and improve
the appearance of ferrous metals, (i.e. iron & steel) as a corrosion
barrier, and then as a sacrificial coating. The application of chromate
conversion coatings over zinc plating, and post-plate sealers, give additional
protection against corrosion particularly under high humidity and moisture
conditions.For additional information
on Chromate Coatings and Post-Plate Sealers, please read, “Heal Thyself! – How Chromates on Zinc Plating
work”, elsewhere in this report.

In dry air, a protective layer of oxide soon forms on an
untreated zinc surface, and subsequent attack is slow.In moist air, zinc hydroxide forms first on
the surface, and is then converted to zinc carbonate.If the surface has not been chromated, the
carbonate takes the form of a bulky, loose layer, often described as white
rust, or wet storage stain.8In confined spaces, zinc is attacked
by organic acid vapors emitted by woods, plastics, and various insulating
materials 8.

Commercially, zinc is deposited in thick nesses ranging from
0.0001”- 0.0005”, depending upon the intended application and the corrosion
protection required, the majority of which is 0.0001”-0.0003”, commonly known
as “Commercial Zinc”.Commercial Zinc
has a high coefficient of friction, low strength, moderate abrasion resistance,
poor impact resistance, brittle at room temperature, but malleable at
212-302°F.

To relieve the potential for Hydrogen Embrittlement in
hardened steels electro-plated with Zinc, a baking procedure after the plating
is required to remove, or diffuse the hydrogen throughout the basis metal,
reducing the risk of embrittlement.For
more information on Hydrogen Embrittlement, please read the section titled, “You
Crack Me Up!Hydrogen Embrittlement is
No Laughing Matter”, located elsewhere in this report.

“Heal
Thyself! – How Hexavalent Chromates on Zinc Plating work”

Hexavalent Chromate coatings exhibit
a phenomenon called “self healing” – the ability to
protect metal on areas where some of the coating has been removed as the result
of a scratch, or an abrasion.Chromate conversion coatings on zinc plated steel are often used as
a final finish to retard the formation of white or gray products of corrosion
of the zinc during environmental exposure, and also prevents surface
discoloration from fingerprints and perspiration as the result of
handling.From this consideration alone,
application of a chromate coating is advisable.

Figure 1

In
general, chromates are applied by immersing the zinc-plated parts in a solution
containing dichromate, or chromic acid and an activator (usually, nitrate,
sulfate, chloride, formate, or fluoride).An oxidation-reduction reaction occurs on the metal surface with the
formation of substrate metal ions and trivalent chromium ions.An accompanying increase in the pH of the
solution immediately adjacent to the metal surface results in the precipitation
of a gelatinous film, comprised largely of chromic hydroxide, and in which
soluble chromates are incorporated.This
freshly formed coating, after rinsing and drying, is rather soft and vulnerable
to damage, but soon hardens in less than 48 hours.In addition, the chromate coating itself also
contributes some protection by presenting a barrier between the metal and the
environment.The protective value of a chromate finish
increases with increasing thickness.The final appearance of the chromate film
depends on the base metal smoothness and the quality of the plated
deposit.The duration of
protection provided by zinc coatings is a function of: coating thickness,
exposure conditions, post plating treatments, and chromate post sealers.

Chromated Zinc:

·Bright Zinc:Single-dip bright 8-24 hours to white corrosion product

·Yellow (Iridescent) Zinc: A typical iridescent
chromate coating prevents the appearance of white salts from corrosion of the
underlying metal for more than 96 hours of salt-spray exposure.

·Black (Bronze) Zinc: 96 hours to white corrosion
product

·Olive Drab Zinc: 120-172 hours to white
corrosion product

Post-Treatment Sealers: After the zinc
plating, and chromate has been applied, a post-plate “sealer” can be applied
that will significantly enhance the corrosion protection.The Sealer chemically bonds with the chromate
film to seal and harden chromate films as well as increase their adhesion to
zinc surfaces.It will also reduce
chromate leaching and fingerprints while dramatically improving corrosion
resistance.Sealers may be applied over
bright (clear), yellow, olive drab, or black chromate conversion coatings.Salt Spray results have shown a 50%-100%
increase in corrosion protection after the addition of a post-plate sealer, and
red rust protection up to 300-500 hours.Additionally, the cost for the sealer can be very economical, especially
when considering the importance of the enhanced corrosion protection provided.

“Can’t Stand the Heat?”

Heating of chromated zinc adversely affects corrosion
resistance, as the heat causes a decrease of the available (leachable)
inhibitive hexavalent chromium by an irreversible dehydration phenomenon and
cracks appear in the surface film.The
adverse effects are worsened as the temperature increases, and after heating to
above 212°F, the protective nature of the chromate film may be nullified.Since zinc plated high-strength steel (above
Rockwell C-40) requires heating to relieve hydrogen embrittlement, the chromating
operation is deferred until after baking.

FAQ’s About Zinc
Plating

1.What
Is Commercial Zinc?Commercial
Zinc is the name or label given to a zinc finish specification that is commonly
used in finishing metal parts. When specifying “Commercial Zinc”, you get a
basic range of zinc finish protection. The normal composition has a thickness
of .0002” of electroplated zinc.Additionally, some commercial zinc formulations add a chromate top
covering to protect the zinc finish.

2.Why Use A Chromate On Zinc?Post-Plate Chromate treatments are
used primarily to improve corrosion resistance, improve paint or adhesive
bonding properties, and provide a decorative or colored finish.

3.What can Post-Treatment Sealers do for zinc
plating? After the zinc
plating, and chromate has been applied, a post-plate “sealer” can be applied
that will significantly enhance the corrosion protection.The Sealer chemically bonds with the chromate
film to seal and harden chromate films as well as increase their adhesion to
zinc surfaces.It will also reduce
chromate leaching and fingerprints while dramatically improving corrosion
resistance.Sealers may be applied over
bright (clear), yellow, olive drab, or black chromate conversion coatings.Salt Spray results have shown a 50%-100%
increase in corrosion protection after the addition of a post-plate sealer, and
red rust protection up to 300-500 hours.Additionally, the cost for the sealer can be very economical, especially
when considering the importance of the enhanced corrosion protection provided.

4.What about Zinc Alloy Plating?There are several alloys of zinc that
are used throughout the industry.The
more common types include Zinc-Cobalt, but others are Tin-Zinc, Zinc-Nickel,
and Zinc-Iron, all of which provide better corrosion protection than zinc
alone.

5.Will E-coat and Paint Adhere To A Zinc Finish?In short, Yes!E-coat and Paint will adhere to a zinc
finish, and provide superior corrosion protection, as the protective value of
the combined finishes provides excellent protection for the base metal.We recommend Zinc with a Yellow chromate to
provide the best adhesion.

6.Why
Is Salt Spray Testing Used?Salt spray testing is a means to
measure the relative protective value of a particular finish. The key word is
relative. By rigidly controlling the exposure environment, a value can be
derived to measure when corrosion starts. The American Society for Testing
Methods (ASTM) salt spray specification B 117-90 is a detailed testing method
for controlling the amount of salt spray solution, at what temperature, in what
direction and much more. The results, normally in hours of exposure, allows for
comparison of different finish formulations.Corrosion resistance of a finish
can be denoted in terms of the number of hours exposed to Salt Spray (Fog)
Testing per ASTM B 117-90 Test Method. The results indicate the number of hours
before white corrosion (the first stage of reaction) begins. Figure 1 reflects
the differences in corrosion resistance abilities of some of the finishes
offered.

7.What
Can I Do To Get More Protection For My Components?First, you must define your application and environment requirements.
What life do you expect from the finish and how should it look after 1, 3, or 5
years?For instance, the finish
protection required for outdoor road use is much more severe than for indoor
office use.Often times, combining
finishes can result in extended protection, such as zinc plating with a
chromate coating, and zinc under E-coat, spray paint, or powder coat.

8.What Are The Different Classes Or Types Of
Zinc Finishes?The American Society
for Testing Methods (ASTM) specification B-633 has four classifications for
electroplated zinc finishes. They are based on coating thickness and type of
application /environment that will be seen. Service Condition 1 is mild indoor
applications and they move up to Service Condition #4 which is VERY SEVERE or
exposure to harsh outdoor high abrasion applications. The basic idea is that
protection increases as the finish thickness increases.

ASTM TypeDescription

Type I ................................................ Zinc,
as plated.........................

9.“What is the difference between Sacrificial
Protection & Barrier Protection?”Sacrificial coatings are those deposits that
give themselves up to the corrosive media, protecting the base metal.Increasing the thickness on sacrificial
coatings extends the life of the protection.Barrier protective coatings (e.g. nickel, e-coat, powder coat, paint,
chrome) are deposits that reduce or eliminate moisture, oxygen, and atmospheric
gases from contacting the base metal. However, unlike sacrificial protection,
any void or break in a barrier coating can lead to an immediate base metal
attack. Therefore, pits or porosity in the base material can be highly
detrimental to a barrier protector.When
a continuous zinc coating is present on steel, for instance, the zinc simply corrodes
at its characteristic rate, which, incidentally, is considerably lower than
that of steel, despite its greater activity in the Galvanic Series.When the plating is discontinuous from pores
or defects in the coating, the exposed steel areas are protected because the
exposed steel becomes the cathodic member of the (steel/zinc) couple where
oxygen discharge occurs and alkalinity is formed.The zinc is, of course, anodic and corrodes
at a faster rate than normal, particularly in the vicinity of the exposed
steel.Any tendency for the steel to
corrode is counteracted by the flow of electrons from the corroding zinc to the
steel surface (cathodic protection).

10.“What are the Limitations of Zinc
Plating?”Zinc should not be used on
critical steel parts that will reach temperatures of 500˚F, or higher, as
zinc may diffuse into grain boundaries to embrittle the steel.Zinc coatings can produce bulky corrosion
products during exposure to marine or tropical environments and should not be
used where the products may cause binding and prevent functioning of equipment
that has moving parts in contact.Rapid
corrosion of zinc can occur in confined atmospheres where repeated condensation
of moisture is likely and where certain organic vapors containing halogen can
accumulate.

Electrodeposited Zinc Technical Data

Characteristics8

Smoothness......................................................................................... High

Brightness............................................................................................ High

What is HE?Have you ever paused to consider when riding
in a car, or walking across a bridge, whether the
plated parts were professionally handled to remove Hydrogen Embrittlement
(HE)?Not paying attention to HE can cause failure of the plated components, resulting in
very serious consequences, or personal injury.When certain steels exposed to sources of hydrogen fracture at stress
levels well below their theoretical strength, HE may be the cause.Steels with hardness above Rockwell C40 are
the most susceptible to HE, including heat-treated
steels used to manufacture many bolts, screws, nuts, springs, lock-washers, and
other fasteners.Do you trust that your
“plater” handles HE properly?

How Does HE Occur?The zinc electroplating process
utilizes electrical energy through the electrical reduction of aqueous
solutions of zinc salts.Because the part is negatively charged to
attract the positively charged zinc ions, it also attracts positively charged
atomic hydrogen ions.Unable to
eradicate hydrogen from our plating processes, we can take precautions to
manage the negative effects of this hydrogen on the parts that are plated.4

Atomic hydrogen moves throughout the metal, following cracks
and impurity lines until it suddenly comes to an open area, or void, in the
crystalline structure encountering zero pressure, and begins to bounce
around.Along comes a second hydrogen atom,
and soon the two collide, readily forming hydrogen gas (H2).As the volume of H2 builds, the
pressure increases because the larger H2 molecule does not readily
move out of the base material, and is “entrapped”.This process continues as more H2
molecules are trapped, and the resulting pressure increase causes the stress
that we are concerned about.Brittle
Fracture occurs when the stress exceeds the yield point of the base material.In practice, problems with HE are rare when
dealing with low-strength steels, but there are a number of problems with
high-strength steels.

How to Solve the HE Problem? To
relieve the potential for HE, a baking procedure after
the plating either removes the hydrogen, or diffuses it throughout the basis
metal, both reducing the risk of embrittlement.Mechanically plated, hardened steel parts, when processed according to
standard procedures, should be held 24 hours before use.If particularly aggressive cleaning
procedures are required to remove excessive amounts of heat-treat scale prior
to mechanical plating, the waiting period should be extended to
48 hours.4

Professionally Managed: Customers
can’t easily check the residual hydrogen plated steel parts, so they must rely
on their “plater” for assurance that hydrogen management practices have been
instituted, including4:

·Customer: Providing proper notations
on part drawings requiring HE bake-out.

·Customer: Identifying on purchase
order documentation the need for HE bake-out.

·Customer: Using a plater that
follows a detailed cleaning and plating procedure.

·Plater: Maintaining and calibrating
oven equipment and controls.

·Plater: Keeping quality records that
are available for review.

In
conclusion, where critical parts are involved and where high-strength steel is
the substrate, the safest approach to eliminating HE is with proper baking by a
professionally managed plating company, commonly known as “HE relief bake out”,
providing you with peace of mind.5